Transcript Slide 1

Introduction to Electrical
Machines
1
Contents
■ Basic construction and principles
■ DC machines
■ Synchronous machines
■ Induction machines
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Objectives
■ When you have studied this chapter, you should:
■ have an understanding of electrical machines
■
■
■
construction.
understand the principles of DC machines.
understand the principles and application of synchronous
machines.
understand the principles and application of induction
machines.
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1. Basic Construction
4
Introduction
One of energy can be obtained from the other form
with the help of converters. Converters that are
used to continuously translate electrical input to
mechanical output or vice versa are called electric
machines.
The process of translation is known as
electromechanical energy conversion.
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Electrical
system
e, i
Electric
Machine
Mechanical
system
T, n
Motor
Energy flow
Generator
•An electrical machine is link between an electrical
system and a mechanical system.
•Conversion from mechanical to electrical: generator
•Conversion from electrical to mechanical: motor
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Electrical
Machines
DC
machine
AC
machine
Synchronous
machine
Induction
machine
•Machines are called AC machines (generators or
motors) if the electrical system is AC.
•DC machines (generators or motors) if the electrical
system is DC.
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Electrical
system
e, i
Coupling
magnetic
fields
Mechanical
system
T, n
Two electromagnetic phenomena in the electric machines:
•When a conductor moves in a magnetic field, voltage is
induced in the conductor.
•When a current-carrying conductor is placed in a magnetic
field, the conductor experiences a mechanical force.
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Electric Machines
Basic Structure
a'
b
c
Rotor
c'
•The structure of an electric
machine has two major
components, stator and rotor,
separated by the air gap.
• Stator:
Does not move and normally is
the outer frame of the machine.
b'
a
Stator
Stator
R
B’
Y’
Rotor
Y
• Rotor:
Is free to move and normally is
the inner part of the machine.
N
S
B
R’
•Both rotor and stator are made of
ferromagnetic materials.
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DC Machines
Construction
10
Induction Machines
Construction
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Synchronous Machines
Construction
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2. DC Machines
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3. Synchronous Machines
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Classification of
AC Rotating Machines
■ Synchronous Machines:
•Synchronous Generators: A primary source of electrical
energy
•Synchronous Motors: Used as motors as well as power
factor compensators (synchronous condensers)
■ Asynchronous (Induction) Machines:
•Induction Motors: Most widely used electrical motors in
both domestic and industrial applications.
•Induction Generators: Due to lack of a separate field
excitation, these machines are rarely used as generators.
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Synchronous Machine
■ Unlike induction machines, the rotating air gap field
and the rotor rotate at the same speed, called the
synchronous speed.
■ Synchronous machines are used primarily as
generators of electrical power, called synchronous
generators or alternators.
■ They are usually large machines generating
electrical power at hydro, nuclear, or thermal power
stations.
■ Application as a motor: pumps in generating
stations, electric clocks, timers, and so forth where
constant speed is desired.
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Synchronous Machines
Generator
Exciter
View of a two-pole round rotor generator and exciter
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Synchronous Machine
■ Round Rotor Machine
•The stator is a ring shaped
laminated iron-core with
slots.
•Three phase windings are
placed in the slots.
•Round solid iron rotor with
slots.
•A single winding is placed
in the slots. Dc current is
supplied through slip
rings.
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Round Rotor Machine
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Synchronous Machine
■ Salient Rotor Machine
•The stator has a laminated
iron-core with slots and
three phase windings placed
in the slots.
•The rotor has salient poles
excited by dc current.
•DC current is supplied to the
rotor through slip-rings and
brushes.
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Salient Rotor Machine
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Synchronous Generator
■ Principle of Operation
1) From an external source, the
field winding is supplied with
a DC current -> excitation.
2) Rotor (field) winding is
mechanically turned (rotated)
at synchronous speed.
3) The rotating magnetic field
produced by the field current
induces voltages in the outer
stator (armature) winding. The
frequency of these voltages is
in synchronism with the rotor
speed.
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Parallel Operation of Synchronous
Generator
■ Generators are rarely used in isolated situations. More
commonly, generators are used in parallel, often
massively in parallel, such as in the power grid. The
following steps must be adhered to:
■ •when adding a generator to an existing power grid:
1) RMS line voltages of the two generators must be the
same.
2) Phase sequence must be the same.
3) Phase angles of the corresponding phases must be the
same.
4) Frequency must be the same.
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4. Induction Machines
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Induction Machine
■ The induction machine is the most rugged and
■
■
■
■
■
the most widely used machine in industry.
Both stator and rotor winding carry alternating
currents.
The alternating current (ac) is supplied to the
stator winding directly and to the rotor winding
by induction – hence the name induction machine.
Application (1f): washing machines, refrigerators,
blenders, juice mixers, stereo turntables, etc.
2f induction motors are used primarily as
servomotors in a control system.
Application 3f: pumps, fans, compressors, paper
mills, textile mills, etc.
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Induction Motors
■ The single-phase
induction motor
is the most
frequently used
motor in the
world
■ Most appliances,
such as washing
machines and
refrigerators, use
a single-phase
induction
machine
■ Highly reliable
and economical
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Induction Motors
■ For industrial
applications, the
three-phase
induction motor is
used to drive
machines
■ Large three-phase
induction motor.
(Courtesy
Siemens).
Housing
Motor
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Induction Machine
■ General
•The induction machine is used
as the most common motors
in different applications.
•It has a stator and a rotor like
other type of motors.
•2 different type of rotors:
1-squirrel-cage winding,
2-Wound-rotor
•Both three-phase and singlephase motors are widely
used.
•Majority of the motors used by
industry are squirrel-cage
induction motors
Construction of Induction Motor
A typical motor consists of two parts:
1-An outside stationary stator having
coils supplied with AC current to
produce a rotating magnetic field,
2-An inside rotor attached to the
output shaft that is given a torque
by the rotating field.
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Induction Motor
■ Basic principles:
■ •An AC current is applied in the
Induction motor components.
stator armature which
generates a flux in the stator
magnetic circuit.
■ •This flux induces an emf in the
conducting bars of rotor as
they are “cut” by the flux while
the magnet is being moved (E =
BVL (Faraday’s Law))
■ •A current flows in the rotor
circuit due to the induced emf,
which in term produces a force,
(F = BIL ) can be changed to
the torque as the output.
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Induction Motor
■ Stator construction
■ –The stator of an induction
Single-phase stator with windings.
motor is laminated iron core
with slots similar to a stator
of a synchronous machine
■ –Coils are placed in the slots
to form a three or single
phase winding.
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Induction Motors Magnetic Circuit
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Squirrel-cage Rotor
■ Rotor is from laminated iron
core with slots.
■ Metal (Aluminum) bars are
molded in the slots instead
of a winding.
■ Two rings short circuits the
bars.–Most of single phase
induction motors have
Squirrel-Cage rotor.
■ One or 2 fans are attached to
the shaft in the sides of rotor
to cool the circuit.
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Induction Motor
■ It is usually for large 3 phase
Compared to squirrel cage rotors,
wound rotor motors are
expensive and require
maintenance of the slip rings and
brushes, so it is not so common
in industry applications
induction motors.
■ •Rotor has a winding the same
as stator and the end of each
phase is connected to a slip ring.
■ •Three brushes contact the three •Wound rotor induction motor was
the standard form for variable
slip-rings to three connected
speed control before the advent
resistances (3-phase Y) for
of motor
reduction of starting current and
speed control.
Rotor of a large
induction motor.
(Courtesy
Siemens).
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Synchronous Speed
ns =
120f
p
ns = synchronous speed [r/min]
f = frequency of supply [hertz/Hz]
P = total of magnetic pole
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Slip and Slip Speed
The slip s of an induction motor is the difference between the
synchronous speed and the rotor speed, expressed as a
Percent (per unit) of synchronous speed
The per-unit slip is given by the equation
S=
ns - nr
ns
S = slip
ns = synchronous speed [r/min]
nr = rotor speed [r/min]
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Voltage and frequency induced in the rotor
The voltage and frequency induced in the rotor both depend
on the slip. They are given by the following equation
f2 = s f
E2 = s Eoc (approx.)
f2 = frequency of the voltage and current in the rotor [Hz]
f = frequency of the source connected to the stator [Hz]
s = slip
E2 = voltage induced in the rotor at the slip s
Eoc = open-circuit voltae induced in the rotor when at rest [V]
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Active Power in a Induction Motor
Efficiency () =
Poutput
Pinput
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Motor Torque
Tm =
9.55 Pm
n
9.55 (1 – s) Pr
=
ns (1 – s)
= 9.55 Pr / ns
Tm = 9.55 Pr / ns
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I2R losses in the rotor
Pjr = s Pr
Pjr = rotor I2R losses [W]
s = slip
Pr = power transmitted to the rotor [W]
Mechanical Power
Pm = Pr - Pjr
= Pr - s Pr
= (1 – s) Pr
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Example 1
Calculate the synchronous speed of a 3-phase
induction motor having 20 poles when it is
connected to a 50 Hz source.
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Knowing quantities:
Source frequency = 50 Hz,
number of poles = 20
120 f
Synchronous speed ns =
=
p
120 x 50
20
ns = 300 r/min
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Example 2
A 0.5 hp, 6-pole induction motor is excited by a 3-phase,
60 Hz source. If the full-load is 1140 r/min, calculate the
slip.
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Knowing quantities:
Source frequency = 60 Hz,
number of poles = 6
Full load/rotor speed = 1140 r/min
120 f
Synchronous speed ns =
=
p
120 x 60
6
ns = 1200 r/min
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Slip speed: ns – n = 1200 – 1140 = 60 r/min
Slip: s = (ns - n) / ns
= 60/1200
= 0.05 or 5%
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Example 3
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Student Assignment 2
A single phase, 4 poles induction motor gives the
following data:
Output 373 W ; 230 V
Frequency : 50 Hz., Input current 2.9 A
Power factor: 0.71 ; Speed: 1410 r.p.m.
a) Calculate the efficiency of the motor
b) Determine the slip of the motor when delivering
the rated output
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